1. Field
This patent specification relates to electromagnetic measurements made in connection with boreholes. More particularly, this patent specification relates to methods and systems for single-well electromagnetic induction measurements through metallic casing.
2. Background
Formation resistivity (or conductivity) is a common measurement for hydrocarbon exploration/detection. A rock formation of given porosity has high conductivity when the pore space contains high brine saturation and low conductivity when the pore space contains hydrocarbons. By quantifying the porosity and resistivity, the hydrocarbon saturation can be estimated (Archies Law).
Downhole conductivity measurements on rock formations are made either by inductive means (a Transmitter coil inducing a current in the formation which in turn generates a secondary field sensed by a Receiver coil) or by ohmic/galvanic means (injecting a current through a couple of electrodes and measuring the Voltage developed across them).
Induction tools are used to measure conductivity of rock formations in Open Hole wells. Tools such as Schlumberger's AIT family and RT Scanner are employed for single-well logging, whereas the DeepLook-EM (Crosswell EM) tool is used to perform measurements between wells.
Schlumberger's DeepLook-EM tool in addition to Open Hole, also performs induction measurements through Casing by deploying a Transmitter string in one well and a Receiver string in another well and applying a casing correction method to remove the casing effects on the measurement.
Resistivity measurements through casing are difficult to make due to the high losses in the transmitted signal through casing and also due to the large casing imprint in the measured signal due to casing heterogeneous (non-uniform) properties (magnetic, conductive and thickness).
Cross-well electromagnetic surveys have been used in the past to map formation in-between two wells in oil-field environments. There are several papers in this area, For example, see the modeling studies described in B. R. Spies and T. M. Habashy, Sensitivity analysis of crosswell electromagnetics, Geophysics, Vol. 60, No. 3, P. 834-845 (1995); and David L. Alumbaugh and H. Frank Morrison, Theoretical and practical considerations for crosswell electromagnetic tomography assuming a cylindrical geometry, Geophysics,. Vol. 60, No. 3, 1995, P. 846-870, and an experimental study described in M. J. Wilt, D. L. Alumbaugh, H. F. Morrison, A. Becker, K. H. Lee and M. Deszcz-Pan, Crosswell electromagnetic tomography: system design considerations and field results, Geophysics, Vol. 60, No. 3, 1995, P. 871-885. More recently, regarding a new generation crosswell EM system, see Luis DePavia, Ping Zhang, David Alumbaugh, Cyrille Levesque, Hong Zhang and Richard Rosthal, Next generation cross-well EM imaging tool, SPE, 2008. It is often the case that at least one of the tool strings in a cross-well measurement application needs to be put into a metallic cased well for data collection within the depths of interest. The inhomogenieties of the well casings, both in thickness, diameters, and electromagnetic properties, make it challenging to remove these casing imprints on EM data in order to get high resolution inversion images. There are some references relating to casing imprints removal. For example, see U.S. Patent Publication No. 2011/0204896, and U.S. Patent Publication No. 2009-0281731 hereinafter referred to as “the '896 application” and “the '731 application” respectively, and both incorporated herein by reference.
The casing correction described in the '731 Patent Application involves using numerical modeling codes to calculate the casing attenuation and coil impedance to build up a look-up table, and then search for the corresponding metallic casing attenuation factor for the measured impedance at given frequencies.
Another tool developed by Schlumberger is the Electromagnetic Imaging Tool (EMIT) which uses induction methods in a single well to evaluate downhole metallic casing conditions by measuring EM thickness at multiple frequencies. U.S. Patent Publication No. 2009/0302852 (hereinafter “the '852 patent application, and incorporated by reference herein) discloses combining crosswell and surface to borehole (STB) surveys with EMIT surveys in magnetic and non-magnetic metallic casings, to remove casing imprints in crosswell data through EMIT measurements of casing thickness combined with numerical modeling calculations of casing attenuations. This method may also be useful for single well through casing induction logging tool.
In single-well through-casing induction-logging technology, the main challenges are how to remove casing imprints in logging data and extract formation information, and how to achieve useful range of depth of investigation. U.S. Pat. No. 5,426,367 is related to induction logging in cased wells.